Startup delay circuits assure the delivery of power to circuit components at the appropriate time. A startup delay circuit may be coupled to a circuit for the purpose of providing an appropriately timed startup signal. These circuits receive an input voltage, and after the expiration of a period of time, generate a signal that is used to initiate the operation of the system that it is coupled to. The signal may be delivered to circuits that rely on the delay to assure that they are fully powered before the startup signal is received.
Integrated circuits are an example of a type of circuit where the delivery of power at the appropriate time is critical. Integrated circuits are conventionally comprised of a plurality of distinct internal sub-circuits that function together as a part of an integrated circuit system. Each of the internal sub-circuits must receive power at the appropriate time in order to operate properly. There are many factors that can impact the proper delivery of power to the sub circuits of an integrated circuit.
Switching regulators are another example of a type of circuit where the proper delivery of power is critical. Often it is necessary for a switching regulator to be prevented from activating its switch until all of its internal circuits are powered up and operational. If its switch (which is connected in series with an inductor) is prematurely activated (before its internal circuits are properly powered) the regulator may be unable to ascertain the time at which it should turn the switch off. If the switch is left on too long, it may cause an excessive flow of current through the switch and through the inductor. This excessive flow of current could cause damage to these devices. Moreover, the excessive flow of current could cause the regulator's output voltage to rise to a level that could damage any circuit connected thereto.
As discussed above, to make sure that a device is not started (the regulator does not begin to switch) until all of its internal circuits have been powered, many conventional devices employ startup delay circuits which will prevent the device from operating (e.g., regulator from switching) until some delay time has elapsed after the devices input voltage has been applied.
An important characteristic of a suitable delay circuit is the capacity to provide sufficient delay time relative to the size of a given resistor capacitor pair. Many conventional delay circuits provide insufficient delay time relative to the size of the resistor and capacitor pair that they employ. Conventional delay circuits that are capable of providing longer delays require larger resistor and capacitor pairs. It should be appreciated that delay circuits that utilize larger components occupy more area on the integrated circuit chips that employ them. Consequently, the fabrication cost of the integrated circuit chip is increased.